1. Field of the Invention
The present invention relates to an improved numerically controlled nibbling machine in which the feed distance of a workpiece is limited by a release time during which a work tool is disengaged from the workpiece according to a predetermined number of strokes of the work tool per unit of time, i.e., the speed of rotation of the ram shaft.
2. Description of the Prior Art
Numerically controlled nibbling machines of the foregoing type are known in the art. See, for example, Siemens - Zeitschrift 1970 and 1973, specifically, the respective Supplements on Numerical Control, pages 74-79 and 70-71, respectively. Such nibbling machines are extensively used at present for the working of sheet metal. During continuous nibbling, the workpiece feed mechanism is released during every stroke by means of end switches disposed in the path of the ram of the nibbling machine, specifically during the time during which the work tool is disengaged from the workpiece. Such nibbling machines operate at speeds of up to 400 strokes per minute and at relatively small clearances of the work tool of a few millimeters. Consequently, feeding of the workpiece is possible only during a very short period of time.
In present controls for such nibbling machines, the feed rate of the workpiece is predetermined by a programmer, and the feed distance travelled per stroke of the work tool is then obtained in conjunction with the respective available release time. Such programming requires a certain amount of calculating effort on the part of the programmer. In addition, large mechanical stresses can occur in the machine as a result of the release and interruption of the feeding of the workpiece which must be adapted to the highest number of strokes and the largest sheet metal thickness.
One solution to the foregoing problems has been proposed in copending U.S. patent application Ser. No. 744,883 filed on Nov. 24, 1976 for "Improved Means for Controlling the Feed Rate of a Workpiece in a Numerically Controlled Nibbling Machine." According to this solution, the foregoing disadvantages are overcome by providing means for determining the release time of the work tool in a single stroke of the work tool for any number of strokes of the tool, and means for automatically determining minimum accelerations and decelerations of the workpiece required for travelling the feed distance and for forming a desired feed rate value for the workpiece from the determined release time of the work tool and the feed distance of the workpiece per stroke.
The problem which now arises is how to stop the ram of the nibbling machine at an upper dead-center position. This can be achieved, for example, by means of an end switch placed in the path of the ram of the machine. If the nibbling machine uses different ram speeds (RPMs), the machine is first slowed down to the lowest rotational speed of the ram shaft. A stop command signal is then generated by the end switch in order to terminate movement of the ram. The disadvantage of this arrangement is that, for safety reasons, the end switch can be released only if the command signal for the lowest rotational speed of the ram shaft has already been generated and has been present for some time and it can be assumed that the ram shaft has slowed down to this lowest speed of rotation.